1. Field of the Invention
The present invention generally relates to Chemical Vapor Deposition (CVD) of aluminum oxide in the manufacture of semiconductor devices and, more particularly, to a removable gas injector design which can facilitate reduced cycle time associated with cleaning of its components and elimination of uncertainties associated with manual cleaning of the prior injector design that was not removable. The invention has important applications to other processes using a warm-wall CVD reactor such as MOCVD (metal oxide chemical vapor deposition), in which Al.sub.2 O.sub.3 is a particular example.
2. Description of the Prior Art
In the Chemical Vapor Deposition (CVD) technique, an important element to create and maintain the chemical vapor atmosphere at the wafer source is the gas injector. The injector receives a number of gases and discretely conveys them to the area above the surface of the wafer or substrate where they mix, react, and then form a layer on the wafer or substrate.
In order to insure a proper film or coating, the gases must be introduced substantially uniformly over the surface of the wafer or substrate. If the gases are not uniformly delivered, they do not mix properly, do not have desirable chemical concentrations, and consequently lead to defective deposition of films or coatings on the wafers or substrates. Therefore, it is important to refine the method and apparatus of delivering gaseous chemicals to a wafer or substrate to achieve a uniform deposition of a film or coating.
During the normal operation of an aluminum oxide low pressure CVD (LP CVD) system, the gas mixing injectors are kept at an elevated temperature both by radiation heating from the substrate holder and by the conduction heating from the attached hot wall process chamber. This elevated temperature is nominally in the range of 150.degree.-200.degree. C. and is suspected to be hotter in localized areas. During deposition cycles of the LP CVD aluminum oxide, the reactant gas (Aluminum Triisopropoxide) contacts the injector surfaces and, due to the elevated temperature, some deposits are formed on these surfaces. See, for example, those injector surfaces identified as `b` in FIG. 3. These deposits can alter the chemistry of gases flowing past them, thus changing the concentrations and species entering the reaction chamber. For example, assume that a delivery tube of stainless steel is heated to 200.degree. C. and Aluminum Triisopropoxide passes through it. As long as the interior surface of the tube remains clean, the Aluminum Triisopropoxide remains pure. However, a thin coating of Aluminum oxide on the interior surface of the stainless steel tube can result in a degradation of gas delivered to include some alcohols. Thus, the channel does not need to be completely sealed to have a negative effect on the film composition and uniformity.
Over a period of time, the deposits can seal off the flow passages in the injector, thus creating a non-uniform flow and mixing pattern. This is manifested by the disruption of film composition, film uniformity, film quality, or a combination of these on the substrate. During subsequent cleaning procedures, especially using in-situ cleaning procedures, there are no means to insure that internal flow passages are cleaned sufficiently to allow unrestricted flow. Moreover, the coatings on the injector can become sources for particulates, and partially cleaned injector passages may become sources for particulate contamination.
In the current system, neither the flow baffle nor the flow injector are removable. Deposits on the baffle plate and the injector cause the following problems: (1) clogging, (2) particulates, and (3) chemistry modification. More specifically, clogging of the injector holes and any small openings in the injector/baffle plate assembly cause a lack of reproducibility from wafer to wafer. The additional buildup of film on the injector and baffle plate components can lead to cracking and flaking of the deposit producing particulates which reduce the yield (i.e., area of good regions) on the substrate surface. Films on the injector and baffle plate can modify the chemistry taking place in the reactor leading to additional film uniformity, film composition, and film quality problems at the substrate and a lack of reproducibility of films from wafer to wafer.
The current procedures are to cool the hot wall process chamber, the source of the delivery system, and shut off the heat to the chuck. Once the system reaches a safe temperature, the flow distribution baffle and the injector are then manually scrubbed with an abrasive material, and efforts are made to clear the hydroxide of Aluminum depositions in the flow passages of the injector and the baffle surface. In the original system, before the injector could be cleaned, the gas delivery system and chamber needed to be purged, at operating temperature, for ninety minutes, and then the heaters would be turned off, allowing the system to reach a safe temperature in about six hours. After the injector was cleaned, it took about eight hours for the system to stabilize at the operating temperature. There was no guarantee that the injector passages had been adequately cleaned due to the complex drilling patterns that cannot be seen visually with a fixed, non-removable injector installed.